This invention relates in general to a wafer processing system in which the wafer is clamped to a platen and relates more particularly to such a system in which the temperature of the wafer is regulated.
In many applications, it is important to control the temperature of the wafer during processing. Excessive heating of a wafer can produce excessive diffusion of dopants within the wafer, can outgas and shrink photoresist that has been patterned to define the limits of wafer features and can segregate impurities at epitaxial interfaces. This problem is increasingly important as linewidths shrink because of the reduced tolerance for undesired diffusion and because of the large number of processing steps that must be performed without degrading the results of earlier steps. Excessive heating can result in numerous process steps including physical vapor deposition, ion implantation, ion beam milling and reactive ion etching.
Demand for high system throughput favors the use of high power ion beams in such steps, thereby producing an undesired rate of wafer heating. Batch process can reduce the amount of heating by keeping throughput high while reducing the power dissipated per wafer. However, the reduction of feature size favors the use of single wafer processing systems. In addition, for acceptable throughput, even batch processing systems can exhibit unacceptable levels of heating. It is therefore important to include in the wafer processing system a mechanism for cooling the wafer during processing.
Conversely, numerous process steps are best performed at elevated temperatures. For example, elevated temperatures are useful in implantation and diffusion steps to assist diffusion of the dopants and healing of the lattice structure. Similarly, step coverage can be improved by utilizing elevated wafer temperatures during deposition steps. It is therefore important to include in the wafer processing system a mechanism for heating the wafer during processing. In particular, such wafer heating can be used to elevate the wafer to the desired process temperature before the process step is initiated.
A number of prior systems have included heating and or cooling of the wafer during wafer processing. Early systems relied on radiative heating and cooling to regulate the temperature of the wafer. Unfortunately, silicon wafers are relatively transparent to infrared radiation and the rates of cooling and heating by radiation alone have been inadequate.
Subsequent systems clamp the wafer to a platen and regulate the wafer temperature by regulating the temperature of the platen. Unfortunately, at the microscopic level, the solid-to-solid contact between a platen and a wafer exhibits actual contact over less than 5% of the interface between the wafer and platen. This is not a significant problem at normal ambient pressures since gas molecules filling the regions between the contact points provide a significant amount of thermal conduction. Unfortunately, the wafer is typically processed at extremely low pressures so that there is only a very small contribution from the gas particles in the spaces between the solid-to-solid contact points. To improve the thermal conductivity between the wafer and the platen, one prior method utilizes a thermally conductive, pliable material between the wafer and the platen.
Unfortunately, as is indicated in U.S. Pat. No. 4,261,762 entitled Method For Conducting Heat To Or From An Article Being Treated Under Vacuum, issued to Monroe L. King on Apr. 14, 1981, this approach exhibits problems with repeatability, thermal nonuniformity and excessively expensive maintenance. Therefore, in this system, the wafer is clamped directly against the platen and a gas is provided to the interface between the wafer and the platen to provide gas-assisted thermal transport between these two elements. This gas is provided through a vertical channel through the platen at a pressure of about 0.5 to 2.0 Torr. The platen is cooled to remove heat from the wafer.
In U.S. Pat. No. 4,743,570 entitled Method Of Thermal Treatment Of A Wafer In An Evacuated Environment, issued to Lawrence T. Lamont, Jr. on May 10, 1988, the platen includes both heating and cooling mechanisms. The wafer is again held in direct contact with the platen by a set of four clamps that grip the wafer by its peripheral edge. However, because these clips are relatively flimsy, the gas pressure between the wafer and platen is limited to the range 0.1-1 Torr.
In U.S. Pat. No. 4,512,391 entitled Apparatus For Thermal Treatment Of Semiconductor Wafers By Gas Conduction Incorporating Peripheral Gas Inlet, issued to David J. Harra on Apr. 23, 1985, the platen includes a lip against which the wafer is held to produce, in the region between the wafer and the platen and surrounded by the lip, a first cavity within which heat is transported between the platen and wafer by a gas. Gas is provided on axis into a second cavity which is connected to the first cavity by a plurality of inlets located just inside of the lip. This structure produces a uniform gas pressure over almost the entire volume of the first cavity, thereby producing an equally uniform thermal conductance.
In U.S. Pat. No. 4,457,359 entitled Apparatus For Gas-assisted Solid-to-solid Thermal Transfer With A Semiconductor Wafer, issued to Scott C. Holden on Jul. 3, 1984, a spring-biased clamp presses a wafer against a domed platen with sufficient force to bend the wafer into conforming contact with the platen. A groove in the platen just inside of the region in which the wafer is clamped, supplies gas to the interface between the wafer and platen. This large amount of prestressing the wafer enables a much larger gas pressure to be produced at the wafer-clamp interface without bowing the wafer away from the platen. When the gas between the wafer and the platen is on the order of or larger than 5 Torr, gas flow is laminar so that there is no increase in thermal conductivity above this pressure.
In U.S. Pat. No. 4,542,298 entitled Methods And Apparatus For Gas-Assisted Thermal Transfer With A Semiconductor Wafer, issued to Scott C. Holden on Sep. 17, 1985, the clamp and an attached bellows function as a seal between the ion bombardment chamber and a region into which gas leaks from between the wafer and the platen. This reduces the the amount of gas leakage into the ion bombardment chamber.
In U.S. Pat. No. 4,671,204 entitled Low Compliance Seal For Gas-enhanced Wafer Cooling In Vacuum, issued to Jon M. Ballou on Jun. 9, 1987, a new type of sealing ring is presented that produces an adequate seal between the wafer and platen without significantly stressing the wafer.